Being on the receiving end of one of these bonds, I believe the general consensus is You have both signed it so you had best both live up to it.
I nosed around the CAB a while back mostly out of curiosity and they seemed to think that a 3 year bond for a course was fairly excessive and could possible be challenged due to it being an unreasonable length of time - but they were very quick to stress that it would be a pretty difficult thing to follow through.
As for signing a bond after a course - sod that, you would be morally obligated to work for the company for a year, but the bond should be signed first, not tagged on at the end
best of luck